To answer this question, we first need to establish what epistolary form is. In a nutshell, this refers to a novel that has been written in the form of letters. In more unusual cases, an epistolary novel can take the form of other documents, such as newspaper clippings or diary entries.

In the case of The Color Purple, Alice Walker has written this novel in the form of a series of letters that the protagonist, Celie, writes to God. If ever there was a character in need of God's help, I would argue that it's Celie. Having endured sexual abuse at the hands her father, she has borne two children who have been taken away from her. She was then forced to marry another abusive older man.

The effect of epistolary form is a view into our protagonist's mind that cannot be surpassed. Since she is often writing to God, who is all-knowing anyway, she has no need to hide any of her innermost thoughts, fears, or desires. In addition, the letters that are exchanged between Celie and her sister, Nettie, provide us with a narrative that is both deeply personal and extremely vulnerable in nature.

In summary, epistolary form is used to great effect in The Color Purple to provide insights into the lives and psyches of subjugated and abused women.

An epistolary novel is one in which the story is told through a series of letters, diary entries, notes and other written forms by which individuals communicate. In other words, an epistolary novel is unconventional in that it does not simply tell a story through the first- or third-person, but rather through one or more characters in the form of written records. Classic examples of such novels include Mary Shelley’s Frankenstein and Bram Stoker’s Dracula. Alice Walker’s The Color Purple is the story of Celie, a desperately poor African American woman whose life is told through the epistolary style. As Walker’s novel begins, Celie relates her story through a series of letters to God. As the novel progresses, and Celie discovers the letters from her sister Nettie, the epistolary form is adapted accordingly and the letters Celie writes are to Nettie while she, in turn, begins to read the letters that Nettie had penned over the preceding years.

The word epistolary refers to literary works that written in the form of letters or journal entries. Therefore, this novel uses the epistolary form because it is comprised of letters that Celie and her sister Nettie write.  

This form has some unique advantages and disadvantages. Because Celie, at least initially, cannot interpret or reflect on her experiences, the audience gets a very candid, almost childlike perspective on them. She's a reliable narrator in the sense that she is extremely honest, like a child, and because she addresses her letters to God (whom she believes would already know everything anyway, so there is no reason for her to lie). Further, it can be difficult to keep track of the novel's timeline since Celie includes no dates and only refers to the passage of time when a character she's quoting makes a reference to it.  This style of narration does bring us extremely close to Celie, rendering her even more sympathetic than she might otherwise be, especially because of her lack of understanding about what's happening to her early in the novel.